Ultrasonics is basically the science of the effects of sound vibrations beyond the limit of audible frequencies. Ultrasonics has been used in a wide variety of applications. For example, ultrasonics has been used for (1) dust, smoke and mist precipitation; (2) preparation of colloidal dispersions; (3) cleaning of metal parts and fabrics; (4) friction welding; (5) the formation of catalysts; (6) the degassing and solidification of molten metals; (7) the extraction of flavor oils in brewing; (8) electroplating; (9) drilling hard materials; (10) fluxless soldering and (10) nondestructive testing such as in diagnostic medicine.
The object of high power ultrasonic applications is to bring about some permanent physical change in the material treated. This process requires the flow of vibratory power per unit of area or volume. Depending on the application, the power density may range from less than a watt to thousands of watts per square centimeter. Although the original ultrasonic power devices operated at radio frequencies, today most operate at 20-69 kHz.
The piezoelectric sandwich-type transducer driven by an electronic power supply has emerged as the most common source of ultrasonic power; the overall efficiency of such equipment (net acoustic power per electric-line power) is typically greater than 70%. The maximum power from a conventional transducer is inversely proportional to the square of the frequency. Some applications, such as cleaning, may have many transducers working into a common load.
Other, more particular areas where ultrasonic vibratory force has been utilized are in the areas of thin nonwoven webs and thin films. For example, ultrasonic force has been used to bond or weld nonwoven webs. See, for example, U.S. Pat. Nos. 3,575,752 to Carpenter, 3,660,186 to Sager et al., 3,966,519 to Mitchell et al. and 4,695,454 to Sayovitz et al. which disclose the use of ultrasonics to bond or weld nonwoven webs. U.S. Pat. No. 3,488,240 to Roberts, describes the use of ultrasonics to bond or weld thin films such as oriented polyesters.
Ultrasonic force has also been utilized to aperture nonwoven webs. See, for example, U.S. Pat. Nos. 3,949,127 to Ostermeier et al. and 3,966,519 to Mitchell et al..
Lastly, ultrasonic force has been used to aperture thin film material. See, for example, U.S. Pat. No. 3,756,880 to Graczyk.
Other methods for the aperturing of thin film have been developed For example U.S. Pat. No. 4,815,714 to Douglas discusses the aperturing of a thin film by first abrading the film, which is in filled and unoriented form, and then subjecting the film to corona discharge treatment.
One of the difficulties and obstacles in the use of ultrasonic force in the formation of apertures in materials is the fact that control of the amount of force which is applied was difficult. This lack of control resulted in the limitation of ultrasonic force to form large apertures as opposed to small microapertures. Such an application is discussed in U.K. patent application number 2,124,134 to Blair. One of the possible reasons that ultrasonics has not found satisfactory acceptance in the area of microaperture formation is that the amount of vibrational energy required to form an aperture often resulted in a melt-through of the film.
As has previously been stated, those in the art had recognized that ultrasonics could be utilized to form apertures in nonwoven webs. See, U.S. patent to Mitchell, et al.. Additionally, the Mitchell et al. patent discloses that the amount of ultrasonic energy being subjected to a nonwoven web could be controlled by applying enough of a liquid to the area at which the ultrasonic energy was being applied to the nonwoven web so that the liquid was present in uncombined form. Importantly, the Mitchell, et al. patent states that the liquid is moved by the action of the ultrasonic force within the nonwoven web to cause aperture formation in the web by fiber rearrangement and entanglement. The Mitchell et al. patent also states that, in its broadest aspects, since these effects are obtained primarily through physical movement of fibers, the method of their invention may be utilized to bond or increase the strength of a wide variety of fibrous webs.
While the discovery disclosed in the Mitchell et al. patent, no doubt, was an important contribution to the art, it clearly did not address the possibility of area embossing of nonfibrous thin thermoplastic films or materials having fibers in such a condition that they could not be moved or rearranged. This fact is clear because the Mitchell et al. patent clearly states the belief that the mechanism of aperture formation depended upon fiber rearrangement. Of course, thin films do not have fibers which can be rearranged. Accordingly, it can be stated with conviction that the applicability of a method for area embossing thin thermoplastic films by the application of ultrasonic energy in conjunction with a liquid at the point of application of the ultrasonic energy to the sheet material was not contemplated by the Mitchell et al. patent. Moreover, the Mitchell et al. patent teaches away from such an application because the patent states the belief that aperture formation (physical effects) requires the presence of fibers to be rearranged.